1. Field of the Invention
The present invention relates to an improvement of an electrical power converter arrangement which converts a DC into an AC or vice versa, and, in particular, relates to an output voltage control of such an electric power converter arrangement.
2. Description of Related Art
B. Velaerts et al. "A Novel Approach to the Generation and Optimization of Three-Level PWM Wave Forms" (PESC '88 RECORD, TEEE, April 1988, pp 1255-1262) proposes a use of a dipolar modulation mode which is adapted to alternatively output positive and negative pulse like voltages via zero potential for improving output voltage wave forms of a three-level inverter.
The above paper indicates that the transition from a dipolar modulation mode to a unipolar modulation mode, which is adapted to output only pulse like voltages having the same polarity as the output voltage, has to be performed at optimum six points existing in one cycle.
Further, FIG. 9 in JP-A-2-101969 (1990), which corresponds U.S. Pat. No. 4,953,069, discloses existence of both the dipolar modulation period and the unipolar modulation period in a half cycle of the output voltage.
The use of the dipolar modulation mode is desirable for an improvement of the output voltage wave forms as well as for a fine voltage regulation.
However, when a certain output voltage is required to be realized, the dipolar modulation mode necessitates having pulses in opposite polarity to the output voltage, thereby the voltage utilization rate decreases. Accordingly, the transition from the dipolar modulation mode to the unipolar modulation mode is inevitable.
When the dipolar modulation mode is shifted to the unipolar modulation mode, and when the load thereof is an AC motor, motor torque fluctuation is caused from increased current ripples during the change-over.
The former prior art indicates that the transition between the dipolar modulation mode and the unipolar modulation mode is controlled to be carried out at a predetermined phase.
However, such control must be carried out after determining the optimum points which exist only six times in one cycle. Therefore the control system, in relation with the timing, increases in control complexity.
Further, for example, when an output voltage of an electric power converter is required in response to a certain output voltage command by making use of the unipolar modulation mode, the feet portions of the sinusoidal output voltage command can not be realized correctly. This is because there exists a minimum on time in the switching elements constituting the electric power converter, and such a small output voltage command as in the feet portions can not be realized as the output voltage of the electric power converter.
Now, FIG. 9 of the latter prior art illustrates a waveform which includes a fixed period (60.degree.) in dipolar modulation mode and the remaining period in unipolar modulation mode in one cycle. However, such waveform is outputted in order that a portion where the modulation wave exceeds over 1 is compensated with the other phases.
Accordingly, in the above prior art, very small output voltage feet portions portions, can not be realized.